A chain tool like this: To remove the broken link before you replace it.

An Allen key multitool perhaps with a chain tool like this to save you carrying a separate one: This multi tool comes with a chain tool and a billion allen keys.

Zip ties for temporary repairs if something breaks or comes loose

Gordon’s portable toolkit is so minimal, you could carry it in a cycling jersey’ back pockets, or in a pannier or rucsack’s outer pouch. Yet this mini kit should enable you to deal with 99% of the ‘mechanicals’ you’re ever likely to encounter.

One final tip

This might sound obvious but check you kit is fit for the bike your riding. Here’s a salutary tale from the Bike Co-op’s Ged.

My Sunday bike (the Roubaix) has QR wheels, and its folding tyres can be removed with one lever. My everyday cross/town bike (the Genesis) has track nuts, which require a 15mm spanner, plus I need two or three levers to remove its ultra tight wire-bead Schwalbe Marathon tyres.

If I go out on the Roubaix I’m sometimes precious about keeping it’s weight low, so I remove what’s not required from the emergency kit. Then I discover a few days later that I don’t have the tools required to fix the Genesis because I neglected to replenish the tool kit. Doh.

Suggestion.
How about an appropriate £3 hanger especially if abroad?.
I did not have this part (OK, not a tool) when i bent hanger 2 days into 10 days thro’ Pyrenees.
Small bike shop managed to straighten it, but over 200 hangers generally available, not surprisingly mine not in stock!
Trip over if he had not managed to sort it for me, but 50:50 chance of success, could have snapped while repairing.
Just a thought.